Method and apparatus for depositing films of refractory metal oxides and refractory metals



Sept. 28, 1965 1. AMES ETAL METHOD AND APPARATUS FOR DEPOSITING FILMS OF' REFRACTORY METAL OXIDES AND REFRACTORY METALS Filed Jan. 5, 1962 FIGZ IRVING AMES By ROYCE SIMMONS Ma, W, mi, 21M

ATTORNEKSv United States Patent Olce 3,208,873 Patented Sept. 28, 1965 METHOD AND APPARATUS FUR DEPOSITING FILMS F REFRACTORY METAL OX'IDES AND REFRA'CTORY META'LS Irving Ames, Peekskill, and Royce G. Simmons, Poughkeepsie, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Jan. 5, 1962, Ser. No. 164,533 .14 Claims. (Cl. 117-106) The present invention relates to systems for the production of volatile gaseous compounds for use in the deposition of films on various substrates. More specifically, the present invention relates to a metal source which permits rapid metal oxide generation and subsequent deposition in the form of a thin lilm of oxide or parent metal depending upon ambient conditions.

Early researchers discovered that the transport of refractory metals results from the oxidation of the metal followed by the volatilization and redeposition of the oxide or reduced metal oxide thus formed. This sequence of reactions was found to take place at a temperature considerably below the temperature required to melt or vaporize the parent refractory metal itself. An example of this phenomenon may be observed in the gradual deterioration of filaments in vacuum tubes wherein the metal filament reacts with oxygen present inside the tube to produce a metal oxide which evaporates and deposits on the walls of the tube which are approximately at room temperature. The metal itself may be deposited on the walls of the tube where there is available ionized hydrogen for reduction of the metal oxide.

Advantage has been taken of this basic concept to produce metal oxide protective coatings on substrates, such as precision tools, mirrors, optical elements and the like. The method comprises heating a filament or sheet of a refractory metal in an evacuated housing in which the substrate is positioned. Oxygen is then admitted to the housing where it reacts with the heated metal to form the metal oxide. The metal oxide is in turn volatilized and deposits on the substrate in the form of a thin protective metal oxide coating.

In attempting to adapt this technique to the production of thin oxide films for use as electrical insulation, it was found that the method was unsatisfactory in many aspects and especially with respect to the rate of deposition. The best results obtained by us, according to the prior art process, achieved a deposition rate of approximately l angstrom per second in film thickness.

Moreover, it is inherent in the prior art process that the entire system be exposed to a relatively high partial pressure of pure oxygen throughout the oxide generation and deposition. Under such conditions, it is possible for oxygen to contaminate the substrate, the deposited lilms and also other metal evaporant sources, where it is desired to carry out multiple depositions in the same chamber.

Accordingly, it is a broad object of the present invention to provide an improved system for the production of volatile gaseous compounds for use in the deposition of films on substrates.

It is a primary object of the present invention to provide a method and apparatus for generating metal oxide in a manner which lends itself to the rapid deposition of the oxide and which also results in the production of pure, uniform and reproducible films.

f A further object of the present invention is to provide method and apparatus for generating vaporized metal oxide whereby a high oxygen concentration is maintained adjacent the metal evaporant source, While a relatively low oxygen concentration is maintained in the balance of the system and especially adjacent to the substrate.

Another object of the present invention is to provide a method and apparatus for generating metal oxide by which the rate of deposition of the vaporized oxide may be readily controlled and monitored.

An additional object of the present invention is to provide apparatus for generating vaporized metal oxide which assists in the deposition of the oxide on a base in a highly accurate and carefully controlled manner and according to any desired pattern.

The present invention is based on the discovery that significantly higher rates of deposition and improved iilms may be produced by contacting a heated source of refractory metal evaporant with a high density of oxygen in a first Zone then introducing the metal oxide formed in the first Zone into contact with the substrate located in a second zone and continuously maintaining an oxygen partial pressure differential between the first zone and the second zone.

The nature of the invention and the manner in which the objects of the invention are achieved are pointed out m-ore fully in the following description and claims and are illustrated in the accompanying drawing which discloses, by way of example, the principle of the invention and the best mode which has been contemplated for applying that principle.

FIGURE 1 is a cross-sectional view of an apparatus in accordance with the invention and which may be employed in carrying out the method of the invention;

FIGURE 2 is a cross-sectional view of an apparatus in accordance with the invention which also may be employed in practicing the method of the invention, and features another embodiment of the evaporant source;

FIGURE 3 is .a cross-sectional view in keeping with the invention and also illustrates another modified form of evaporant source; and

FIGURE 4 is a cross-sectional view illustrating an apparatus in keeping with the present invention by use of which metal films, rather than metal oxide iilms, may be produced.

The following general description of the method and apparatus of the present invention, with reference to the illustrative embodiments set forth in the drawing, will assist a clear understanding of the novel features of the invention and of the manner in which the stated objects of the invention and many other highly useful objects and advantages of the invention are achieved.

According to the present invention, and referring to FIGURE l of the drawing, a substrate 10 which is to be coated with a film derived from metal oxide vapors is placed in a housing or chamber 11 which is composed of a base element 12 and a cover 13. The cover 13, as shown, is a dome or bell-shaped transparent member of high strength glass. A pump, not shown, connected with vacuum line 14 is continuously operated during the deposition and is preferably operated so as to maintain a pumping speed of several hundred liters per second.

The source of metal which is to be reacted with oxygen to provide the vaporized metal oxide for deposition on substrate 10 is in the form of tubular element 15. This element may be formed from any of a number of refractory metals which react with oxygen to produce volatile metal oxides useful as coating films either in the oxide state or reduced to the parent metal. Metals such as tungsten, molybdenum, tantalum, vanadium, titanium, chromium, thorium, zirconium and niobium are satisfactory for use in accordance with the present invention. Molybdenum, which has been found to be particularly useful, since it yields highly volatile oxides, is employed in the following examples, it being understood that any readily oxidizable metal can be so employed.

Tubular refractory metal element 15 is seated in a tubular metal sleeve 16 which serves as a support for element 15 and as a conduit for the introduction of oxygen fromanexternal supply, not shown. Sleevev16n may be rigidly or removably mounted in base 12 so Vas to communicate with the exterior of chamber 11.

Sets of conductors 17 are attached by means of contacts 18 to tubular element 15. Contacts 18 are clamped to a fin which is welded to tube 15. Conductors 17 are in turn connected to an external source of current, not shown, for the resistive heating of the refractory metal tube.

In operation, current is passed through conductors 17 and contacts 18 whereby tubular refractory element 15 is heated to a temperature between 1000 C. and l500 C., the exact temperature depending on the melting point of the metal forming the tube. When the tube has reached the desired temperature, oxygen is introduced through -the sleeve 16 and into contact with, the inside walls of tube 15. By virtue of this c-ontact, the oxygen reacts with the metal walls of the tube forming volatile metal oxides which exit into chamber 11 .and deposit in the form of a thin film on substrate 10.

By evacuating chamber 11 through line 14 at a rate of several hundred liters per second a high differential is obtained between the partial pressure of oxygen wi-thin the confines of tubular element 15 and the partial pressure of oxygen in the deposition chamber 11 adjacent substrate 10.

By carrying out the oxide generation and deposition in this mannerit has been found that rates of deposition of up to forty-tive angstroms per second in film thickness may be achieved as opposed to approximately one angstrom per second with prior art techniques. Also, the relatively low partial pressure of oxygen outside of the zone in which the reaction between the metal and oxygen takes place, i.e., within the confines of tubular element 15, avoids oxygen contamination of the substrate and the deposited film and results in more uniform deposition and higher purity films.

It should also be observed thatmeasurement of the partial pressure of oxygen in the deposition zone provides a convenient means for monitoring the rate of deposition. Once a standard system has been calibrated, such a determination can be made since the partial pressure of oxygen inthe deposition chamber is inversely proportional to the rate of metal oxide formation and the rate of oxide formation is in turn directly proportional to the rate of film deposition.

To further assist in an understanding of the invention, the following detailed example, with reference to FIG- URE 1 of the drawing, may be of assistance:

Oxygen is admitted at a rate low enough to prevent the pressure within the bell jar from rising above (x10-4) mm. Hg during the use of the source. The oxygen is admitted through sleeve 16 into contact with the tubular molybdenum element heated to a temperature of approximately 1300" C. The molybdenum element has an internal diameter of approximately 3A; inch. Chamber 11, having a capacity of 40 liters, is continuously evacuated so as Ito maintain a partial pressure of oxygen within tubular element 15 of approximately 2 mm. Hg and a partial pressure of oxygen in the rest of the system of approximately 3 X10-4 mm. Hg. Deposition is continued for 3 minutes during which period a film of molybdenum oxide of about 6000 A, in thickness is developed on a 2" x 2" substrate located in chamber 11 several inches laway from the top of the molybdenum tube and maintained at a temperature of approximately 20 C.

As shown in FIGURE 2 of the drawing, the method and apparatus` of the present invention may also be adapted to the production of films of highly accurate configurations and carefully controlled dimensions according to any desired pattern. The apparatus illustrated in FIGURE 2 comprises a metal evaporant source having a tubular body of refractory metal 25 provided with a conical nozzle 26 of the same metal which defines a restricted orifice 27. In order to facilitate heating of the nozzle portion of the metal evaporant source, an annular filament 28, the cathode of an electron bombardment system, is positioned around the tip of the nozzle. Conventional resistive heating leads 17 may be attached to main bodyV portion of tubular element 25 through contacts 18 to accomplish heating of the main portion of the evaporant source.

The substrate 30 to be coated in the embodiment of the apparatus shown in FIGURE 2 is in the form of a running length of work taken from supply reel 31, drawn past aperture 27 and rewound upon take-up or storage reel 32. The longitudinal and horizontal movement4 of the substrate may both be controlled to yield film coatings of any desired configuration or pattern and the accuracy of the deposit is greatly enhanced by use of nozzle 26.

In the embodiment shown in FIGURE 2 of the drawing, it is desirable to maintain a minimum distance between the aperture 27 and substrate 30 so that maximum control of the dimensions of the deposited film may be achieved. By the same token, it is undesirable to permit heat radiated from metal evaporant source 25 to irnpinge on the surface of the substrate, since the substrate itself or the deposited film may be adversely affected by such heating. Therefore, a radiation shield 33 mounted on vertical shield supports 34 is positioned between lament 28 and the substrate 30. The radiation shield may be in the form of a plate of insulating material having a central annular aperture closely surrounding the top of nozzle 26 and effectively blocking the radiation of heat from the evaporant source 25 to the substrate 30.

The mode of operation of the apparatus illustrated in FIGURE 2 is substantially the same as the operation of the apparatus of FIGURE 1 already discussed in the preceding paragraphs. Oxygen is admitted through tubular evaporant source 25 which is resistively heated by the current supplied by wires 17 and electron bombardment cathode 28.v The oxygen within the tube reacts with the metal to produce metal oxide vapors which exit through aperture 27 into contact with the relatively cool substrate 30 whereupon deposition occurs. As already noted, substrate 30 may be moved horizontally and longitudinally'past the aperture 27 of nozzle 26 in order to control the configuration of the deposit on the substrate.

Again, a high oxygen partial pressure differential is maintained between the zone of reaction inside evaporant source 25 and the zone of deposition in which the substrate 30 is situated. The segregating effect of the restricted aperture or orifice 27 assists in the maintenance of the desired pressure differential in this embodiment of the invention.

FIGURE 3 of the drawing illustrates a further embodiment of the evaporant source in keeping with the present invention. The evaporant source here illustrated comprises a hollow, porous plug 40 of refractory metal mounted in a block of copper or other conducting metal 41. Recess 42 and passageway 43 in the copper block 41 communicate with an oxygen supply line 44 so that oxygen may be introduced behind the hollow, porous plug 40. Metal plug 40 is heated by electron bombardment from annular cathode ring 45. Cathode ring 45 and copper block 41 areV connected by electrical conductors 46 and 47 respectively to an external source of potential which furnishes the necessary source of energy for heating metal plug 40 by electron bombardment. Since the copper block 41 may also be subjected to considerable heating in this arrangement of apparatus, a cooling fluid is introduced through duct 50 and withdrawn through duct 51 to prevent overheating of copper block 41. Block 41 may also be insulated from tube 44 and plug 40.

Also, by employing an annular cathode ring in proximity to the conical porous metal plug, heating is concentrated principally at the tip of the plug so that most of the oxide formation takes place at this point resulting in a substantially single point source of oxide vapors.

In operation, the chamber 11 is evacuated through line Z0, porous metal plug 40 is heated to the desired temperature by electron bombardment from cathode ring 45 and oxygen is introduced through conduit 44 into passage 43 and thence into recess 42 in copper block 41. Circulation of cooling fluid through conduit 50 is also commenced. Upon contacting heated metal plug 40, the oxygen forms metal oxides which are volatilized through the porous plug into the deposition zone and into contact with the substrate for production of the desired oxide lm.

As mentioned briefly earlier in the description, the metal oxide vapors generated in accordance with the present invention may be employed to produce metal oxide films or metal films. In the latter case, the metal oxides are reduced adjacent to the substrate to produce a film of the parent metal. FIGURE 4 illustrates an embodiment of the present invention useful in producing metal films in accordance with the invention. The apparatus is substantially identical to that illustrated and described in connection with FIGURE l of the drawing, except that a source of hydrogen ions is provided adjacent to the surface of the substrate. The source of hydrogen ions comprises a conduit 60 connected to an external source of hydrogen ions, not shown, and a nozzle 61. Nozzle 61 is arranged to direct a stream of hydrogen ions across the surface of the substrate to effect reduction of the metal oxides generated within tubular element 1S and passing from that element into the zone surrounding the substrate. Operation of the apparatus illustrated in FIGURE 4 is substantially identical to the mode of operation described in connection with FIGURE 1 with the exception that sutiicient hydrogen ions are introduced next to the substrate surface to effect complete reduction of the metal oxides to yield the desired metal film, rather than a metal oxide film.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to preferred embodiments, it will be understood that various omissions and substitutions and changes in form and details of the device illustrated and in its operation may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A method for depositing a coating onto a substrate comprising:

( l) heating a refractory metal to a predetermined temperature in a lirst zone,

(a) said refractory metal being one which is capable of reacting with oxygen at said predetermined temperature to form refractory metal oxide vapors, and

(b) said predetermined temperature being below the melting point of said refractory metal,

(2) contacting said refractory metal at said predetermined temperature in said rst zone with oxygen to generate rafractory metal oxide vapors,

(3) contacting a substrate with said rafractory metal oxide vapors in a second zone,

(a) said substrate being maintained at a temperature below the condensation temperature of said refractory metal oxide vapors thereby causing the condensation of said vapors and the deposition of a film of refractory metal oxide on the surface of said substrate, and

(4) continuously maintaining said second zone at an oxygen partial pressure substantially lower than the oxygen partial pressure maintained in said first zone.

2. The method of claim 1 wherein the refractory metal is molybdenum.

3. The method of claim 1 wherein said second zone is maintained at an oxygen partial pressure substantially lower than the oxygen partial pressure maintained in said rst zone by continuously evacuating said second zone.

4. A method for depositing a coating onto a substrate comprising:

(1) heating a refractory metal to a predetermined temperature in a first zone,

(a) said refractory metal being one which is capable of reacting with oxygen at said predetermined temperature to form refractory metal oxide vapors,

(b) said predetermined temperature being below the melting point of said refractory metal, and

(c) said refractory metal forming at least part of a fluid conduit,

(2) passing oxygen through said fluid conduit into contact with said refractory metal heated to said predetermined temperature to generate said refractory metal oxide vapors,

(3) conducting said refractory metal oxide vapors into contact with a substrate in a second zone,

(a) said substrate being maintained at a temperature below the condensation temperature of said refractory metal oxide vapors thereby causing the condensation of said vapors and the deposition of a film of refractory metal oxide on the surface of said substrate, and

(4) continuously maintaining a substantially lower partial pressure of oxygen in said second zone than the partial pressure of oxygen in said fluid conduit.

5. The method of claim 4 wherein the refractory metal is molybdenum.

6. The method of claim 4 wherein said second zone is maintained at an oxygen partial pressure substantially lower than the oxygen partial pressure maintained in said first zone by continuously evacuating saidy second zone.

7. The method for depositing a coating onto a substrate comprising:

(1) heating a refractory metal to a predetermined temperature in a rst zone,

(a) said refractory metal being one which is capable of reacting with oxygen at said predetermined temperature to form refractory metal oxide vapors, and

(b) said predetermined temperature being below the melting point of said refractory metal,

(2) contacting said refractory metal heated to said predetermined temperature in said first zone with oxygen to generate refractory metal oxide vapors,

(3) conducting said refractory metal oxide vapors to a second zone in which said substrate is positioned,

(4) reducing said refractory metal oxide vapors with hydrogen ions in said second zone to produce refractory metal vapors,

(5) contacting said substrate with said refractory metal vapors,

(a) said substrate being maintained at a temperature -below the condensation temperature of said refractory metal vapors, and

(6) continuously maintaining an oxygen partial pressure in said second zone substantially lower than the oxygen partial pressure maintained in said first zone.-

8. The method of claim 7 wherein the refractory metal is molybdenum.

9. The method of claim 7 wherein said second zone is maintained at an oxygen partial pressure substantially lower than the oxygen partial pressure maintained in said rst zone by continuously evacuating said second zone.

10. Apparatusv for thedeposition of coatings on substrates comprising:

(1) a pressure-resistant coating substrate is positioned,

(2) a fluid conduit communicating with the interior of said coating chamber,

(a) at least a portion of said fluid conduit being formed of a refractory metal capable of reacting with oxygen at a temperature below the melting point of said refractory metal to produce refractory metal oxide vapors,

(b) said fluid conduit being adapted to conduct said vapors into contact with said substrate,

(3) a source of oxygen communicating with said fluid conduit,

(4) means for heating saidy refractory metal portion of said fluid conduit to a temperature at which it is capable of reacting with oxygen to form refractory metal oxide vapors, and

(5) means for maintaining a substantially lower oxygen partial pressure in said coating chamber than in said fluid conduit. e e

11. The apparatus of claim wherein said fluid conduit comprises a tubular refractory metal element having at least one aperture communicating with the interior of said pressure-resistant coating chamber and at least one other aperture communicating with a source of oxygen.

12. The apparatus of claim 11 wherein said tubular refractory metal element is provided with a nozzle for conchamber in which a 8, trolling the diffusion of refractory metal oxide vapors into the interior of saidy coating chamber.

13. The apparatus of claim 10 wherein said fluid conduit comprises a vapor permeable refractory metal member separating said source of oxygen from the interior of said coating chamber, said refractory metal oxide vapors being capable of diffusing through said refractory metal member into the interior of said chamber.

14. The .apparatus of claim 10 further comprising means for feeding into the interior of the chamber a second gas capable of reacting with said refractory metal oxide vapors.

References Cited by the Examiner UNITED STATES PATENTS 2,599,156 6/52 Bousman 117-1072 2,671,034 3/54 Steinfeld 117-107.1 2,904,452 9/59 Reichelt 117-106 2,996,418 8/ 61 Bleil 117-106 FOREIGN PATENTS 726,048 8/ 42 Germany.

OTHER REFERENCES Da Silva et al.: Fabrication of Aluminum Oxide Films, IBM Technical Disclosure Bulletin, vol. 4, No. 6, December 1961, pages 6 and 7.

RICHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner. 

1. A METHOD FOR DEPOSITING A COATING ONTO A SUBSTRATE COMPRISING: (1) HEATING A REFRACTORY METAL TO A PREDETERMINED TEMPERATURE IN A FIRST ZONE, (A) SAID REFRACTORY METAL BEING ONE WHICH IS CAPABLE OF REACTING WITH OXYGEN AT SAID PREDETERMINED TEMPERATURE TO FORM REFRACTORY METAL OXIDE VAPORS, AND (B) SAID PREDETERMINED TEMPERATURE BEING BELOW THE MELTING POINT OF SAID REFRACTORY METAL, (2) CONTACTING SAID REFRACTORY METAL AT SAID PREDETERMINED TEMPERATURE IN SAID FIRST ZONE WITH OXYGEN TO GENERATE REFRACTORY METAL OXIDE VAPORS, (3) CONTACTING A SUBSTRATE WITH SAID REFRACTORY METAL OXIDE VAPORS IN A SECOND ZONE, (A) SAID SUBSTRATE BEING MAINTAINED AT A TEMPERATURE BELOW THE CONDENSATION TEMERPATURE OF SAID REFRACTORY METAL OXIDE VAPORS THEREBY CAUSING THE CONDENSATION OF SAID VAPORS AND THE DEPOSITION OF A FILM OF REFRACTORY METAL OXIDE ON THE SURFACE OF SAID SUBSTRATE, AND (4) CONTINUOUSLY MAINTAINING SAID SECOND ZONE AT AN OXYGEN PARTIAL PRESSURE SUBSTANTIALLY LOWER THAN THE OXYGEN PARTIAL PRESSURE MAINTAINED IN SAID FIRST ZONE. 